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Ecotoxicology and Environmental Safety 56 (2003) 180189
Case study: bioavailability of tin and tin compounds
Heinz Ru del
Fraunhofer Institute for Molecular Biology and Applied Ecology (Fraunhofer IME), 57377 Schmallenberg, Germany
Received 20 March 2003; accepted 20 March 2003
Abstract
This article reviews the literature related to the bioavailability of tin, inorganic tin compounds, and organotin compounds. On the
one hand, the toxicity of metallic tin and inorganic tin compounds is low. In aqueous systems, the potential bioavailability of tin
seems to depend on the concentration of the truly dissolved ion species. Some studies suggest that tin is an essential trace element forhumans. However, organotin compounds have been proven to be of toxicological relevance. Triorganotin compounds are
particularly toxic explaining their wide use as biocides (e.g., in antifouling paints or pesticides). Persistence of organotin compounds
is governed by moderate to fast aerobic biotic degradation processes, slow anaerobic biotic degradation, slow abiotic degradation by
photolysis, and fast, but reversible, adsorption/desorption processes. Organotin compounds are ubiquitously distributed in aquatic
organisms. Bioconcentration in organisms and ecotoxicity are dependent on the bioavailable fraction. The bioavailability is highest
at neutral and slightly alkaline pH and is reduced in the presence of dissolved organic carbon. The biomagnification of organotin
compounds via the food chain is of minor importance compared with the bioconcentration from the water phase.
r 2003 Elsevier Inc. All rights reserved.
Keywords: Availability; Bioavailability; Bioconcentration; Ecotoxicity; Monitoring; Organotin compounds; Tin; Toxicity; Tributyltin (TBT)
1. Introduction
Human beings have used tin since the Bronze Age.
For thousands of years, tin and tin alloys were used for
production of such consumer products as tin dishes or
drinking mugs. Starting with the Industrial Revolution,
inorganic tin compounds were produced for various
purposes. Around 1940 the industrial production of
organotin compounds started. The latter are currently
technically and economically important, for example,
as biocides and plastic stabilizers.
The primary intention of this article is to discuss the
bioavailability of tin and its compounds in the environ-
ment. Emphasis is placed on the organotin compounds,
which are of high toxicological relevance and for which
the database is most extensive. Depending on environ-
mental conditions, organotin compounds exist as
neutral ion pairs and complexes or as cations in aquatic
systems. Therefore, different concepts are applicable
for evaluation of their bioavailability.
This review uses the definitions for the terms
availability and bioavailability of metal ions in aquatic
systems given by Di Toro et al. (2001). They define
availability as the fraction of the total metal in the
water column or sediment compartments that is un-
bound, free, or available for uptake by an organism.
Bioavailability refers to the fraction of the total metal
that is taken up by an organism and subsequently
transported to a site of action/receptor, or target organ.
However, other authors (e.g., Hare, 1992; Mackay and
Fraser, 2000) use the term bioavailability in a broader
sense, comparable to the availability definition ofDi
Toro et al. (2001). Availability may also be designated
as potential bioavailability.
A definition of bioavailability of pollutants in soils
and sediments was given byPeijnenburg et al. (1997a, b
and elsewhere in this issue). These authors suggest
bioavailability to be a dynamic process with the
distinction of two different phases: a physicochemically
driven desorption and a physiologically driven uptake
process. Although this definition was developed for soils
and sediments, it may be applicable to the aquatic phase
as well, because most compounds are adsorbed to
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particles in the water phase (e.g., to dissolved organic
matter, DOM) and are not truly dissolved. For such
truly dissolved compounds (free ions, nonadsorbed
organic compounds), only the second step would be
relevant. The concept of the two subsequent phases has
some similarities with the availability/bioavailability
approach ofDi Toro et al. (2001).From the cited definitions it is obvious that bioavail-
ability is not a scaled property, specific to a certain
compound. It can only be estimated by comparison
of definite experimental situations or in relation to the
bioavailability of other compounds. For example,
bioavailability of a compound in a compartment may
be determined in relation to an ecotoxicological end-
point or to bioaccumulation in an organism. Because
of the high experimental expenditure for these types
of investigations, in many studies a pragmatic approach
is taken to estimate bioavailable fractions. Because a
number of aquatic studies for different compounds
had shown that a correlation exists between the
concentration of freely dissolved compounds in the
water phase and bioconcentration and effects in
organisms, it is assumed that the fraction of compounds
that is freely dissolved is potentially bioavailable
(as discussed for organic chemicals in Haitzer et al.,
1998, or for metal compounds in Di Toro et al., 2001).
However, the presence of DOM and other factors
may reduce the fraction of the compounds that is
freely dissolved (Haitzer et al., 1998; Di Toro et al.,
2001).
The dissolved fraction in aquatic studies is often
determined operationally as the fraction of a compoundthat passes through a 0.45-mm membrane filter. This
approach is also used in international standards for the
determination of metals in water (e.g.,ISO 11885, 1996)
or for the quantification of dissolved organic carbon
in water (EN 1484, 1997). The exact measurement of
the truly dissolved fraction is possible only with high
experimental expenditure or for some compounds
with special techniques (e.g., free copper ions with an
ion-selective electrode). For soils and sediments, bioa-
vailable fractions are often correlated with certain
extraction procedures (with aqueous solutions or
organic solvents). Therefore, extractability is sometimes
used synonymously for bioavailability, or attempts are
made to correlate bioavailability for an organism with
an extraction procedure with a certain solvent (e.g.,Reid
et al., 2000).
This article discusses in separate sections the bioavail-
ability of metallic tin, inorganic tin compounds, and
organotin compounds. In case of the organotin com-
pounds most data are available for tributyltin (TBT).
This literature review is mainly based on information
compiled byBulten and Meinema (1991)for tin and tin
compounds in general, and byFent (1996)andMaguire
(1996) for organotin compounds.
2. Analysis of tin and organotin compounds
A prerequisite for bioavailability studies is a sensitive
and precise analytical method. In case of tin speciation
analyses are necessary to distinguish among the several
tin species. After dissolution of tin and tin alloys in acids
and acid digestion of inorganic tin compounds inenvironmental or biological samples, the analysis may
make use of atomic absorption spectrometry (AAS).
Other methods are inductively coupled plasmaoptical
emission spectrometry (ICPOES), inductively coupled
plasmamass spectrometry (ICPMS), or at low con-
centration levels by hydride generation coupled to AAS
or to ICPMS. Solid samples may be analyzed directly
by X-ray fluorescence spectrometry. Organotin com-
pounds are mostly analyzed after extraction or digestion
applying either the Grignard method (e.g., derivatiza-
tion with pentylmagnesium bromide), or the ethylborate
method. The resulting derivatives from both methods
are volatile and can be analyzed by gas chromatographic
(GC) methods and detection with atomic emission
detection (GC-AED) or mass spectrometry (GC-MS).
A comprehensive technical report of the IUPAC
Commission on Microchemical Techniques and Trace
Analysis on the determination of tin species in the
environment was published recently (Leroy et al., 1998).
Other articles on organotin analysis includeDirkx et al.
(1995), Morabito et al. (2000), and Pellegrino et al.
(2000). Quevauviller et al. (2000) reported on measure-
ments of organotin compounds in environmental
reference materials (mussels, sediments). A number of
certified reference materials for organotin compoundshave been produced (e.g., by BCR or NIST) that allow
the validation of laboratory methods and internal
quality control.
3. Bioavailability of metallic tin and tin alloys
The world smelting output of tin in 1990 was
B225,000 metric tons per year (Graf, 1996). Tin is
generally considered to be nontoxic in its metallic form.
Cases of poisoning with tin are almost unknown,
because ingested tin is poorly absorbed by organisms.
However, massive inhalation of tin by exposed indus-
trial workers may lead to irritation of the respiratory
tract (Graf, 1996).
Although the author knows of no studies on the
bioavailability of metallic tin, it can be assumed that the
bioavailability is low. Only small amounts of tin may be
(chemically) dissolved from tin materials or tin alloys in
the environment (e.g., by acid waters). The resulting tin
ions may then be bioavailable (refer to the next section).
Small amounts of tin also may be dissolved in food in
contact with tin or tin-plated materials and become
potentially bioavailable.
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4. Bioavailability of inorganic tin compounds
A large number of inorganic tin compounds are
known. In these tin compounds the tin atom is either
divalent (stannous) or tetravalent (stannic). Examples
for industrially important compounds are tin(II) chlor-
ide and tin(II) sulfate, which are used for the plating ofsteel; tin(II) fluoride as a compound in some toothpastes
(to prevent tooth decay); tin(IV) oxide in combination
with other pigments as a ceramic colorant; and tin(IV)
chloride as a basic compound in organotin syntheses
(Graf, 1996; Bulten and Meinema, 1991). The world
consumption of inorganic tin is assumed to be o40,000
metric tons per year (Graf, 1996).
Concentrations of inorganic tin in air, soil, and waters
are usually low (except in areas with minerals containing
high levels of tin or in the surroundings of tin processing
industries). Tin toxicities of natural origin in plants,
animals, or humans have not been reported (Bulten and
Meinema, 1991). In aqueous solutions tin(IV) is more
stable than tin(II), which can be oxidized to tin(IV).
Evaluation of inorganic salts of tin revealed only low
toxicities to organisms. Ingested amounts of inorganic
tin salts are only poorly adsorbed (low bioavailability).
For example, only 2.85% of tin(II) and 0.64% of tin(IV)
were absorbed in experiments. This small part entering
the organism was later completely eliminated via the
urine (Bulten and Meinema, 1991).
Only few ecotoxicological studies with inorganic tin
compounds are published. Generally, the toxicity seems
to be low. With an EC50 (i.e., the effect concentration,
where 50% of tested organisms show an effect) of22 mg/L, only a low toxicity was observed with daphnids
when investigating the immobilization (B200 times
higher concentrations of Sn2+ were necessary in
comparison with the EC50 of Cu2+; Khangarot and
Ray, 1989). A study byPawlik-Skowronska et al. (1997)
found that tin(II) and tin(IV) salts inhibited the growth
of a planktonic cyanobacterium. Toxicity grew with
increasing tin concentrations, augmenting both pH
values and test duration; tin(II) seemed to be more
toxic than tin(IV). The presence of humic acids reduced
the toxicity of tin. At high pH values, anionic tin species
like SnO3H, SnO
3
2, or Sn(OH)6
2 exist, while at
neutral or acidic pH values cationic or neutral tin
species like Sn(OH)+, Sn(OH)22+, Sn(OH)2, or SnO are
present (Pawlik-Skowronska et al., 1997). The free Sn2+
ion is not stable at the high pH values tested (stability
occurs only below BpH 4;Pettine et al., 1981).
It is not clear if the toxicity of tin(II) and tin(IV) ions
can also be described using the free-ion activity model
(FIAM;Morel, 1983; Peijnenburg, 2003 this issue). This
model is used to understand the bioavailability of metal
ions and is based on the assumption that the toxicity of
metal ions in water is correlated with the concentration
of the free metal ions and not to the concentration of the
total metal ion fraction, which also includes ions
adsorbed to or complexed by particulate matter or
DOM. In general, the actual concentration of the free
metal ion is mainly dependent on the water parameters
pH, hardness, and DOM. This was demonstrated for
copper byErickson et al. (1996), although exceptions to
this model are known (Campbell, 1995). Thus it is clearthat the bioavailable fraction of tin as of any other metal
ions is not a constant but is multifactorially influenced.
Some studies suggest that tin is an essential trace
element for humans (possibly as an ionic constituent of
gastrine, a stomach-stimulating peptide hormone).
Natural foods contain trace amounts of tin. It is
assumed that the average daily intake is in the range
0.21 mg (Bulten and Meinema, 1991). In feeding
experiments levels of 0.52 ppm of tin in the diet
improved growth of rats by 2560%. Chloride, sulfate,
and orthophosphate salts of tin had no toxic effects in
rats after feeding diets with 450650 ppm tin for 13
weeks. Inorganic tin does not induce teratogenic or
carcinogenic effects (Bulten and Meinema, 1991).
It is assumed that under certain environmental
conditions methylation of inorganic tin by microorgan-
isms takes place (Gadd, 2000). The occurrence of
methyltin compounds in estuarine and coastal environ-
ments was monitored byAmouroux et al. (2000). In the
past some authors doubted that natural methylation of
tin by microorganisms occurs (Bulten and Meinema,
1991).
5. Bioavailability of organotin compounds
5.1. Properties
Organotin compounds are chemicals that possess
at least one tincarbon bond. The tin atom is tetravalent
in all organotin compounds produced industrially. The
general formula for these organotin compounds is
R(4n)SnXn with n 023: The organic groups R arealkyl or aryl groups that are bound covalently with the
central tin atom. X represents such anions as OH,SH, OSnR3, or OR
0. Important chemicals are the
mono-, di- and trisubstituted butyltin and phenyltin
compounds (MBT, DBT, TBT, MPT, DPT, TPT).
Because of the hydrocarbon substituents, organotin
compounds are hydrophobic. The extent of hydropho-
bicity depends on the degree of alkylation/arylation at
the central tin atom (number of groups, length of alkyl
chain).
The water solubilities of most organotin compounds
are low and dependent on pH, ionic strength, and
temperature. Data for TBT-Cl are in the range from 5
to 50 mg/L, whereas the water solubility of DBT-Cl2 is
higher, up to 92 mg/L (Reincke et al., 1999). Depending
on environmental conditions, organotin compounds
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in antifouling systems on ships by January 2003, and a
complete prohibition by January 2008.
Organotin compounds are released through several
routes into the environment (Fent, 1996). The major
input of triorganotin compounds into aquatic systems
derives from their use in antifouling paints. TBT is used
mostly in so-called self-polishing copolymers thatrelease TBT continuously. Harbor areas are especially
affected by TBT contamination. In harbor sediments,
flakes of antifouling paints from the removal of old
coatings may be present and may serve as reservoirs that
cause locally high concentrations of TBT. For other
compounds such as triphenyltin, the input via their use
as pesticides in agriculture is more important. Waters
may be contaminated with organotin compounds by
effluents from industrial plants. Further inputs to the
environment result from the large-scale use of polyvinyl
chloride (PVC), which contains mono- and diorganotin
compounds as stabilizers. Leachate from landfills where
organotin-containing wastes are dumped may contain
organotin residues, as well as municipal wastewater and
sewage sludge. However, in view of the low water
solubility of most industrially produced organotin
compounds and their strong tendency to adsorb to
sediments, substantial widespread surface water con-
tamination from these sources is unlikely (Bulten and
Meinema, 1991). Further, as a result of the similar
strong adsorption to soil, leaching from and transport in
soil do not take place to a measurable extent (Bulten and
Meinema, 1991). From the organotin compounds only
the methyl derivatives are considerable volatile (e.g.,
tetramethyltin). Further, for bis(tributyl)tin oxide(TBTO), a co-distillation with water may occur (Bulten
and Meinema, 1991). Releases into the environment
from waste incineration seem to be of minor importance
(emission products probably are inorganic tin com-
pounds). In general, the contamination of the atmo-
sphere with organotin compounds is assumed to be
low. A more detailed description of releases into the
environment is given in Fent (1996).
Concentrations of organotin compounds in organisms
are particularly high near sources such as commercial
ports, pleasure-boat marinas, shipyards, much-traveled
shipping routes, as well as industrial manufacturers and
processing plants of organotin compounds. In areas free
from sources the loads are lower, with TBT concentra-
tions o10 mg/kg wet wt. (Tanabe et al., 1998). In coastal
areas as well as deep sea, organotin compounds in
organisms are detectable.
As stated above, only a few studies on the bioavail-
ability of organotin compounds are available. However,
numerous monitoring studies demonstrate that organo-
tin compounds are bioavailable in marine and limnic
systems (refer to the data compilation by Maguire,
1996). As an example, data from a monitoring study
using samples from the German Environmental Speci-
men Bank (ESB; Ru del et al., 1999) are presented in
Table 1. ESB samples are taken on a regular basis from
representative ecosystems in Germany following stan-
dard operating procedures (BMU, 2000;UBA, 1996).
The TBT concentrations in marine biota from the
ESB sampling sites of the North Sea remained nearly
constant between 1985 and 1998. In the rivers a time-dependent decrease was obvious, which probably is a
result of the 1990 ban on TBT-containing antifouling
paints for small boats. For TPT a clear decrease was
observed in the marine samples between 1985 and 1998.
This decrease is correlated to the cessation of use of TPT
as a co-toxicant in antifouling paints in 1985. Such a
decrease was not observed in the Rhine and Elbe rivers.
Here the entry of TPT seems to be correlated to the use
of TPT as fungicide (e.g., for application on potatoes).
The concentration data from different trophic levels
of the North Sea suggest that there seems to be no
biomagnification of organotin compounds in this
ecological system. Sta b et al. (1996) have reported
similar results for a limnic ecosystem; see later.
5.4. Bioaccumulation
There are various pathways by which an organism
may take up organotin compounds. The uptake from
the water or sediment phase via the body surface is
referred to as bioconcentration. Uptake via the food
chain is designated as biomagnification. Accumulation,
the result of both pathways, is often proportional to the
concentration of the compound in the environment. The
extent of bioaccumulation is further influenced bybiodegradation/excretion mechanisms of the respective
organism. Bioconcentration factors (BCFs) for organo-
tin compounds vary considerably, most likely as a result
of different environmental conditions and different
taxonomic groups. BCFs for TBT range from o1 up
to 152,000 (range of data as cited inRu del et al., 1999).
Highest BCFs were observed when very low concentra-
tions of TBT were applied in the test systems. DBT and
MBT showed a lower tendency to bioaccumulation.
Biomagnification of organotin compounds over the
food web has also been examined. For the crab
Rhitropanopeus harrisii, the enrichment of TBTO in
the organism was investigated after exposure via water
or via food (as cited in Alzieu, 1996). After 4 days of
feeding with contaminated food, the biomagnification
factor amounted to 4400 in the hepatopancreas and
between 500 and 1300 in other tissues. When dosing via
water, the BCFs were lower by factors of 1030. For the
common mussel Mytilus edulis, a BCF of 5000 was
observed with TBT uptake via the water phase, whereas
a biomagnification factor of only 2 was calculated upon
feeding with contaminated algae (Laughlin et al., 1986).
Mensink et al. (1997) calculated that the North Sea
whelkBuccinum undatumenriches TPT in relation to the
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mussel Mytilus edulis by a factor of B8. In contrast,
TBT concentrations were lower in the snail by a factor
ofB7 as compared with mussels (no biomagnification).
Sta b et al. (1996) analyzed butyltin and phenyltin
compounds in the food web of Netherlands limnic
waters. In the examined waterbirds (top predators), they
found lower TBT and TPT concentrations than in the
species from lower levels of the trophic system such as
fish, mussels, and crustaceans. Thus, there seemed to be
no biomagnification in this ecological system. In general
the availability/bioavailability of organotin compounds
via the food chain seems to be of minor importance for
TBT and TPT as compared with uptake via the water
phase.
5.5. Ecotoxicity
In comparison with inorganic tin compounds, some
organotin compounds are highly toxic. The toxicity of
the different organotin compounds is related to ex-
posure concentration and duration, bioavailability, and
the sensitivity of the organisms. The endocrine disrup-
tion properties of TBT and TPT in certain aquatic
organisms are of major concern. Observed endocrine
effects are pathomorphological transformations of the
genital organs (designated as superimposed sex or
imposex). Endocrine effects were observed at levels of
B1ng/L TBT (Gibbs and Bryan, 1996). TPT is
suspected to have a similar potential disruptive endo-
crine effect (Horiguchi et al., 1997;UBA, 2000).
The chronic toxicity of TBT is also high. The German
Federal Environmental Agency (Umweltbundesamt;
UBA, 2000) uses the following data for their assessment
of TBT and TPT. For a 90-day fish test the no
observed effect concentration (NOEC) was reported to
be 10 ng/L (freshwater fish Poecilia reticulata). Studies
with rainbow trout showed NOECs of 24mg/L after 28
days. TPT toxicity seems to be similarly high. Effect
concentrations were 3.9 mg/L (LC50, i.e., lethal concen-
tration for 50% of exposedPimephales promelaslarvae)
and 0.15mg/L (early life stage test). Effects on plankton
or oysters were observed for TBT at ng/L concentration
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Table 1
Concentration ranges for different organism groups and different sampling sites
Biota/origina Minimum concentration
(mg/kg ww)
Maximum concentration
(mg/kg ww)
Years Comment
TBT
Brown algae/North Sean 16 2 6 19851996 No trend
Common mussels/North Sean 18 4 21 19851996 No trendEel pout/North Sean 5 11 22 19941998 No trendSeagull eggs/North Sea n 3 o1b 4 19941998 No trendZebra mussel/Rhinen 4 4 14 1996 Concentrations increased
downstream
Bream (muscle)/Rhinen 8 11 37 19961998 Concentrations increaseddownstream and decreased
with time
Zebra mussel/Elben 1 940 1996 Site near harborBream (muscle)/Elben 17 25 470 19931998 Concentrations increased
downstream and decreased
with time
TPT
Brown algae/North Sean 16 o5b 14 19851996 Concentrations decreased
with timeCommon mussels/North Sean 18 o5b 98 19851996 Concentrations decreased
with time
Eel pout/North Sean 5 27 60 19941998 Concentrations decreasedwith time
Seagull eggs/North Sea n 3 o5b o5a 19941998 No trendZebra mussel/Rhinen 4 o5b 10 1996 Concentrations decreased
downstream
Bream (muscle)/Rhinen 8 o5b 53 19961998 Concentrations increaseddownstream and with time
Zebra mussel/Elben 1 15 1996 Site near harborBream (muscle)/Elben 17 o5b 253 19931998 Concentrations increased
downstream
Data are given as mg/kg of the respective organotin cation and refer to wet weight (ww).
Source: Environmental specimen bank (Ru
del et al., 1999).an=number of analyses.bBelow the respective limit of determination.
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levels (Alzieu, 1996). For a more detailed discussion of
effects and compilations of data refer to Alzieu (1996)
and Fent (1996).
5.6. Toxicity
The lowest documented toxicological endpoint ofbutyltin compounds is a depression of the immune
system of the thyroid gland by TBTO. The World
Health Organization (WHO) gives a lowest observed
adverse effect level (LOAEL) of 0.25 mg/kg body
weight per day (WHO, 1999). With a safety factor of
10, a no observed adverse effect level (NOAEL) value
of 0.025 mg/kg body weight per day was calculated by
the German Federal Institute for Health Protection of
Consumers and Veterinary Medicine (BGVV, 2000).
With a safety factor of 100, a tolerable daily intake of
0.25mg/kg body weight was stated. Because for DBT no
complete assessment is possible, the authority uses
preliminarily the same total daily intake value as for
TBT until a better data basis becomes available (BGVV,
2000).
Data on the toxicity of organotin compounds to
humans are available from accidental exposures. The
methyl compounds are particularly toxic (Bulten and
Meinema, 1991). In a recent Chinese report of deadly
poisonings after meals prepared from methyltin-con-
taminated food (Gui-Bin et al., 2000), analyses of the
inner organs of one victim yielded high concentration
levels of methyltins in liver (1.93 mg/g DMT, 1.42 mg/g
TMT), kidney (1.05mg/g DMT, 0.47 mg/g TMT), sto-
mach (0.104mg/g DMT, 0.304mg/g TMT), and heart(0.1 mg/g DMT, 1.48mg/g TMT). The respective con-
centrations for a control person were below the limit of
detection. Concentrations of methyltin compounds in
the contaminated food were not given.
Kannan et al. (1999) analyzed the concentrations of
organotin compounds in the blood of 32 Americans
of different origin and age. The mean concentrations
detected were 8 ng/mL MBT (present in 53% of the
samples), 5 ng/mL DBT (81%), and 8 ng/mL TBT
(71%). Levels ranged from concentrations below the
limit of determination up to 101 ng/mL of total butyltins
in blood. The authors assume that the residues are due
to exposures of humans to organotin compounds as
stabilizers or as biocides in household articles. The
toxicological relevance of the observed contamination
levels are unknown.
A similar human monitoring study was recently
conducted with blood samples from the Environmental
Specimen Bank/Human Specimen Bank in Germany.
Only low levels of organotin compounds were detected
(Ru del and Steinhanses, 2001). The investigation
comprised blood samples from student collectives from
two German cities. From the 30 samples analyzed, only
one sample showed a concentration above the limit of
determination for DBT (1.6 ng/mL; limit of determina-
tion, 0.4 ng/mL blood). In 5 samples MBT was found
at levels above the limit of determination (range: 0.7
1.4 ng/mL; limit of determination, 0.3ng/mL blood).
Concentrations of the other compounds analyzed were
below the respective limits of determination
(TBTo0.3 ng/mL; TPTo0.4 ng/mL; DPTo0.3 ng/mL;MPTo 1 ng/mL).
5.7. Factors determining bioavailability
Generally, the parameters that significantly influence
the bioavailability of metals in natural waters and
sediments are hardness, alkalinity, pH, temperature,
oxidation/reduction potential, composition and concen-
tration of other ions, particulate matter, and organic
carbon content. Cation exchange capacity also might
influence the bioavailability in soils and sediments. For
metal ions the pH is the most important factor
controlling partitioning (Di Toro et al., 2001). The most
important phases for interactions with metal ions are the
organic carbon and the metal oxides of the sediment or
soil. For organic compounds the most decisive para-
meter is the organic carbon content in the respective
compartment. In case of polar compounds the pH value
may also be important. For organotin compounds only
a few systematic studies are available on the influences
of pH values and organic matter content on bioavail-
ability.
Fent (1996) and Looser et al. (1998) presented data
on the influence of the pH value on the bioavailability
of organotin compounds in aquatic test systems. Theystudied the uptake and bioconcentration of TBT-Cl and
TPT-Cl in Daphnia magna, fish larvae of Thymallus
thymallus, and the sediment organism Chironomus
riparius. BCFs were B2000 for the fish larvae, 680 for
the sediment organism, and 220 for the daphnids. A
correlation to the lipid content of the organisms was not
found. An important result of the study was that the
BCFs of TBT were higher at pH 8 as compared with pH
5. This correlates with the stability of the neutral TBT
complexes or ion pairs such as TBT-OH or TBT-Cl. The
difference in BCF was expected to be larger when
assuming the octanol-water partition coefficients deter-
mined for the compounds at those pH values (Arnold
et al., 1997). Therefore, it can be assumed that besides
the neutral TBT-Cl/TPT-Cl the TBT/TPT cations were
also taken up by the organisms. However, the neutral,
nondissociated molecules seemed to be more bioavail-
able than the respective organotin cations. Fent (1996)
assumed that the neutral TBT-OH can penetrate
biomembranes more easily than the charged hydrophilic
cations. Another study with fish yielded similar results.
Tsuda et al. (1990) found that the bioaccumulation of
TBT and TPT in carp was higher at more alkaline pH
values as compared with pH values o7.
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As discussed earlier, organotin compounds such as
TBT and TPT exist as neutral nondissociated molecules
at pH values above their pKavalue of 56.5. Because the
pH value of many limnic waters (e.g., in Germany) is
47 and of seawater 48, it is assumed that large
fractions of TBT and TPT in aquatic systems are in the
nondissociated form and therefore are potentiallybioavailable.
In addition to pH, another important variable
influencing bioavailability that was investigated in detail
for organotin compounds is the presence of organic
matter. For organic chemicals,Haitzer et al. (1998)have
compiled a comprehensive review on the influence of
DOM on the bioconcentration. At higher concentra-
tions of DOM, decreases of bioconcentration of 298%
in relation to the controls were found. In contrast, at
low levels of DOM some studies found inexplicable
increases in bioconcentration. Looser et al. (1998) and
Fent (1996) presented evidence that increasing humic
acids concentrations also caused a decrease in biocon-
centration of TBT and TPT. They found that low
concentrations of dissolved humic acids led to small
reductions in the bioconcentration of TPT in daphnids
and fish larvae. At concentration levels of410 mg/L
dissolved organic carbon, the reduction of bioconcen-
tration was significant. For T. thymallus larvae, for
example, the BCF was reduced by nearly 50% by
increasing the organic carbon content from 2 to 13 mg/
L. Further, the uptake times increased, resulting in
longer periods until equilibration was achieved. Fent
(1996) interpreted the results in the way that TBT
interacts with the humic acids, which affects its chemicalspeciation and partitioning. TBT and TPT may form
complexes with carboxyl, amino, or thiol groups of the
organic matter. The observed behavior of the organotin
compounds is similar to the general finding that the
binding of an organic chemical to the organic carbon
phase depends mainly on its hydrophobicity.
An important aspect of the study of Fent (1996) is
that because the applied organic carbon contents (DOM
levels) are those usual in ambient waters, it can be
assumed that only a portion of TBT/TPT is freely
dissolved. The other fraction is bound in organic
complexes and therefore assumed to be not available
for uptake via epithelial surfaces of gills or skin of biota.
The complexes may be too large or too polar to cross
cell membranes (Fent, 1996). Therefore, similar to metal
ions, only the freely dissolved fraction of the organotin
compounds seems to be potentially bioavailable.
6. Conclusion
Tin exists mainly in the oxidation states Sn(0), Sn(II),
and Sn(IV). The toxicity of metallic tin and inorganic tin
compounds on the one hand is low. Some studies even
suggest that tin is an essential trace element for humans.
Organotin compounds, on the other hand, are of high
toxicological relevance. The triorganotin compounds
are especially toxic, which is the reason for their wide
use in antifouling paints or as fungicides. Persistence of
organotin compounds is governed by moderate to fast
biotic degradation processes under aerobic conditions,slow anaerobic biotic degradation, slow abiotic degra-
dation especially by photolysis, and fast but reversible
adsorption/desorption processes. Bioconcentration in
organisms and ecotoxicity of the compounds are related
to the bioavailable fraction of the organotin com-
pounds. Generally, bioavailability is influenced by such
actual environmental conditions as ion composition of
the aqueous environment, pH, dissolved organic carbon
content, and presence of competing compounds. The
bioavailability of organotin compounds is influenced
mainly by the pH value and by the presence of humic
acids/dissolved organic carbon. Bioconcentration fac-
tors of both TBT and TPT were higher at alkaline pH
values as compared with acidic pH values, and increas-
ing humic acids concentrations caused a decrease in
bioconcentration of TBT and TPT in aquatic organisms.
Effect data are available for a large number of
organisms and organotin compounds. Some organisms
including certain marine snails show sensitive endocrine
effects (imposex) at ng/L concentration levels of
triorganotin compounds such as TBT and TPT.
Monitoring data reveal that organotin compounds are
ubiquitous in aquatic organisms, proving the bioavail-
ability of organotin compounds in the environment.
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